Users as first class entities in the system, with prominent profile pages, including such features as: age, sex, location, testimonials, or comments about the user by other users.
The ability to form connections between users, via links to other users who are “friends,” membership in “groups” of various kinds, and subscriptions or RSS feeds of “updates” from other users.
The ability to post content in many forms: photos, videos, blogs, comments and ratings on other users’ content, tagging of own or others’ content, and some ability to control privacy and sharing.
Other more technical features, including a public API to allow third–party enhancements and “mash–ups,” and embedding of various rich content types (e.g., Flash videos), and communication with other users through internal e–mail or IM systems.
RSS - most web 2.0 sites and services offer RSS (really simple syndication) feeds. We'll learn more about this and blogs & wikis and so forth tomorrow, but here is a helpful video and other basic resources in case you want to check it out now.
Web 2.0
Lists of web 2.0 tools: